The invention relates to an apparatus for furnishing a reaction mixture for a reforming catalyst of a fuel cell assembly and to a method for operating such an apparatus.
As legal requirements in terms of pollutant emissions in vehicles have become more stringent, the importance of fuel cells for automotive use has increased markedly. In principle, the emission of greenhouse gases such as carbon dioxide can be prevented entirely. Fuel cells are currently operated on the basis of hydrogen. However, at present the problem of hydrogen storage in the vehicle has not been solved satisfactorily. As an alternative, hydrogen can be generated directly where it is to be used, by reforming or partial oxidation of hydrocarbons (methane, gasoline, diesel, methanol). Although such a process necessarily also creates carbon dioxide, markedly higher efficiency is nevertheless achieved by the direct conversion of the chemically bound energy into electrical energy.
In use, so-called PEM (polymer electrolyte membrane) fuel cells have proven especially advantageous. Such fuel cells tolerate only very slight traces of carbon monoxide in the fuel gas, however, so that typically the fuel cell is preceded by suitable oxidation converters. For hydrogen generation, a reaction mixture is passed through a reforming catalyst, for instance based on noble metal, that is located in a reactor. The reactor usually has a catalytically coated honeycomb structure or a catalyst bulk fill, in order to speed up the chemical conversion of the reaction mixture.
The actual reforming process at the reforming catalyst, depending on the composition of the reaction mixture, comprises a complex interplay between reversible and irreversible redox reactions. The reaction mixture comprises air, hydrocarbons and water.
For automotive use of fuel cells, considerable technical problems exist in addition to the actual reforming process. For instance because of a dynamic power demand by the vehicle driver, an increased hydrogen flow rate is necessary. Furnishing an adequate hydrogen flow rate immediately after cold starting has been equally unsatisfactorily solved. These problems are due in particular to the fact that the furnishing of water for the reaction mixture is done in the prior art in the form of water vapor. In cold starting, the requisite means for generating the water vapor, such as water vapor generators, are not yet at an adequate operating temperature. The water infeed is therefore inadequate. In the event of an increased power demand to the fuel cell, the water vapor flow rate can be increased only with a marked delay. To remedy this, hydrogen storage media are known, which in the travel situations described furnish hydrogen, but also involve disadvantages such as poor operating safety, greater weight and volume, and a high price.
The disadvantages of the prior art can be overcome by the apparatus according to the invention for furnishing a reaction mixture for the reforming catalyst and the method for operating the apparatus. The apparatus comprises
a) an air infeed with means for regulating an air flow rate;
b) a fuel infeed with means for regulating a fuel flow rate;
c) a water vapor generator with means for regulating a water vapor flow rate;
d) a liquid water infeed with means for regulating a liquid water flow rate; and
e) a control unit, which communicates with the liquid water infeed, the water vapor generator, the fuel infeed, and the air infeed.
Because via the control unit, controlling variables for the fuel infeed, the liquid water infeed, the water vapor generator and the air infeed are furnished as a function of a power demand to the fuel cell and as a function of an operating phase of the apparatus, the cold starting and dynamic performance can be quickly adapted to actual requirements.
Advantageously, the liquid water infeed is increased both in the cold starting phase and upon an increase in the power demand to the fuel cell. For infeeding fuel and liquid water, the apparatus has respective regulating valves, which can be triggered electromagnetically, hydraulically or pneumatically. As a spray insert, such regulating valves preferably have a swirl insert or a multiple-hole insert for generating a spray, in order to produce a reaction mixture that is as homogeneous as possible.
The infeeding of the fuel and of the liquid water can be done spatially separately from one another or directly in the same region, either in the inflow line to the reactor or directly in the reactor. It has proved especially preferably to feed both components directly into the reactor, since here the liquid water infeed can be simultaneously for cooling a valve seat of the regulating valve of the fuel infeed.
In a further, especially advantageous feature of the latter arrangement of the fuel and water infeeding, the water cooling in the region of the liquid water infeed has a switching valve or an adjustable throttle valve for setting the water pressure. In this way, the pressure required to generate the spray can be generated in the region of the spray insert. For such an application, it has proved to be advantageous to use a diaphragm spring element to seal the reactor. By prestressing the diaphragm springs, the desired opening pressure, at which a formation of water vapor bubbles is prevented and the requisite atomization quality can be achieved can be set. For metering purposes, a metering pump, proportional valve or clocking valve is accordingly always required in the inflow. For cooling purposes, a switching valve or proportional valve can be provided in the outflow of the needle seat of the fuel valve.
The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of preferred embodiments taken in conjunction with the drawings.